1. Field of the Invention
The present invention relates to a structure for carrying out irradiation while scanning a linear laser beam, and to a structure for annealing a non-single crystal semiconductor film through irradiation of a linear laser beam while scanning the laser beam in a beam width direction. The present invention also relates to a semiconductor device, and to a method of manufacturing the semiconductor device.
2. Description of the Related Art
In recent years, extensive studies have been made on techniques for obtaining a crystalline semiconductor film (semiconductor film having crystallinity of single crystal, or polycrystal, microcrystal, etc.) by annealing a non-single crystal semiconductor film (not a single crystal semiconductor film such as an amorphous, polycrystalline, or microcrystalline semiconductor film) formed on an insulating substrate of glass etc. to crystalize the film or to improve its crystallinity. A silicon film is often used for the above semiconductor film.
As compared with a quartz substrate that has been conventionally frequently used, the glass substrate has such advantages that it is inexpensive, it is superior in workability, and a large substrate can be easily formed. This is the reason why the above-mentioned researches are carried out. Further, the reason why a laser is preferably used for crystallization is that the melting point of the glass substrate is low. The laser is capable of giving high energy to only the semiconductor film without varying the temperature of the substrate very much.
Since a crystalline silicon film formed by performing a laser annealing to a silicon film has high mobility, it is extensively used in such a manner that thin film transistors (TFTs) are formed with this crystalline silicon film, and are employed for, for example, a monolithic liquid crystal electrooptical device in which a TFT for driving pixels and a TFT for driver circuits are formed on one glass substrate. Since the crystalline silicon film is made of a large number of crystal grains, it is also called a polycrystalline silicon film or a polycrystalline semiconductor film.
A method in which a pulse laser beam of an excimer laser etc. with a high power output is optically converted into a square spot of several cm or a linear shape of several mm in width x several tens cm in length on a surface to be irradiated and the laser beam is scanned (irradiation position of the laser beam is moved relatively to the surface to be irradiated) to perform a laser annealing, is superior in mass production and is excellent in industry, so that the method is used by preference.
Particularly, if the linear laser beam is used, contrary to the case where a spot-like laser beam requiring to perform scanning in lengthwise and crosswise directions is used, laser irradiation to all the surface to be irradiated can be performed by scanning in only a direction normal to a line direction of the linear laser, so that a high mass production property can be obtained. The reason why scanning is performed in the direction normal to the line direction is that it is the scanning direction with the highest efficiency. Because of this high mass production property, usage of a linear laser beam obtained by converting an excimer laser beam through a suitable optical system has come to be the mainstream in the laser annealing nowadays.
Generally, in the case where the linear laser beam is formed, an originally rectangular beam is converted into a linear shape through a suitable lens group. The aspect ratio of the rectangular beam is about 2 to 5, and the rectangular beam is deformed into the linear beam with an aspect ratio of 100 or more through, for example, a lens group (this is referred to as a beam homogenizer) shown in FIGS. 2A and 2B. FIGS. 2A and 2B are an upper view and a sectional view, respectively, which show a conventional optical system for forming a linear laser beam. The foregoing lens group is designed such that the distribution of energy in the beam is also uniformed at the same time as the deformation. The method of uniforming the energy distribution is such that the original rectangular beam is divided into parts, and then, the divided parts are respectively enlarged and are overlapped to perform uniforming.
The apparatus shown in FIGS. 2A and 2B has a function of irradiating, as a linear beam, a laser light from an oscillator 201 (in this state, the light has a substantially rectangular shape) through an optical system designated by 202, 203, 204, 205, and 207. Reference numeral 206 designates a mirror.
The cylindrical lens array 202 has a function of dividing a beam into many parts. The divided many beams are synthesized by a cylindrical lens 205 into one.
This structure is needed to uniform the strength distribution in the beam. The combination of the cylindrical lens array 203 and the cylindrical lens 204 has a function similar to the combination of the cylindrical lens array 202 and the cylindrical lens 205.
That is, the combination of the cylindrical lens array 202 and the cylindrical lens 205 has a function of uniforming the energy (strength) distribution of the linear laser beam in the longitudinal direction, and the combination of the cylindrical lens array 203 and the cylindrical lens 204 has a function of uniforming the energy (strength) distribution of the linear laser beam in the width direction. When the cylindrical lens 207 is disposed through the mirror 206, a narrower linear laser beam can be obtained.
An optical system functioning to uniform an energy distribution in a beam is called a beam homogenizer. The optical system shown in FIGS. 2A and 2B is also one of beam homogenizers. A method of uniforming the energy distribution is such that an original rectangular beam is divided into parts, and then, the divided parts are respectively enlarged and are overlapped to perform uniforming.
Some problems have occurred when a laser annealing is applied to a non-single crystal semiconductor film by scanning a pulse laser beam converted into a linear shape. One of the problems is that the laser annealing can not be performed uniformly over the whole film surface according to conditions of the non-single crystal semiconductor film, for example, a film thickness.
In fabrication of a semiconductor device using a semiconductor film, there is a case where the thickness of the semiconductor film is made to be changed in accordance with the properties of a semiconductor component or device to be fabricated. For example, for obtaining high performance, a thin film with a thickness of, for example, about 25 to 55 nm is necessary. Alternatively, for obtaining high reliability, a thick film with a thickness of, for example, about 55 nm to 100 nm is required. Thus, according to the characteristics required for the semiconductor component to be formed, the film thickness of the semiconductor film is made to be changed.
For example, in the case where a non-single crystal semiconductor film with a thickness of 50 nm or less, rather than a non-single crystal semiconductor film with a thickness in the range of 50 nm to 60 nm, is irradiated with a laser beam, a phenomenon where stripes are formed at beam overlapping portions becomes conspicuous, and there is a case where semiconductor characteristics of the film become extremely different among the respective stripes (see FIG. 1).
Even in the case where a non-single crystal semiconductor film having a thickness of 60 nm or more is similarly subjected to a laser annealing, a phenomenon where stripes are formed at overlap portions between a beam and a beam can occur.
For example, in the case where a semiconductor device, for example, a thin film transistor is fabricated by using a crystalline semiconductor film in which the stripes are formed, and a liquid crystal display having such thin film transistors is fabricated, there occurs a disadvantage that the stripes directly appear on a screen display. Although this problem has been remedied by improving the film quality of a non-single crystal semiconductor film as an object to be irradiated by a laser or by fining a scanning pitch (interval between adjacent pulses) of the linear laser, it has not been satisfactory. According to experiments by the present applicant, it was most suitable that the scanning pitch was about one tenth of the beam width of the linear laser beam.
An object of the present invention is to provide a method of carrying out a laser annealing with sufficient uniformity and high productivity in a wide thickness range of a non-single crystal semiconductor film, and to provide a semiconductor device using a crystalline semiconductor film fabricated by such a method.
In order to solve the problems described above, according to one aspect of the present invention, there is provided a laser irradiation apparatus for carrying out irradiation while scanning a linear laser beam having a beam width comprising a first beam in a beam width direction, characterized in that the laser beam on an irradiation surface has a first energy density in a first beam width and a second energy density in a second beam width, and the second energy density is higher than the first energy density.
In the laser irradiation apparatus, the first beam width may be equal to the second beam width.
In the laser irradiation apparatus, scanning may be carried out from a side of the laser beam having the first energy density.
Alternatively, scanning may be carried out from a side of the laser beam having the second energy density.
Further, scanning is carried out from a side of the laser beam having the first energy density, and then, scanning is carried out from a side of the laser beam having the second energy density. Conversely, scanning may be performed from a side of the laser beam having the second energy density, and then, scanning may be performed from a side of the laser beam having the first energy density.
Further, more preferably, a difference between the first energy density and the second energy density is made to 4% to 30% of the first energy density.
Further, it is preferable that irradiation of the laser beam is carried out in an atmosphere of one selected from the group consisting of He, Ar, N2 and a mixed gas of those.
According to another aspect of the present invention disclosed in the present application, there is provided a beam homogenizer comprising: an optical lens for functioning to divide a laser beam; and an optical system for synthesizing laser beams divided by the optical lens, characterized in that the optical lens includes a cylindrical lens and a semi-cylindrical lens (or it can be called half-cylindrical lens).
According to the homogenizer described above, a linear laser beam having the first energy density and the second energy density can be obtained.
The semi-cylindrical lens (or half-cylindrical lens) described above has a shape of one of two congruent solids obtained by dividing a cylindrical lens such that a sectional shape in a lengthwise direction becomes rectangular.
The optical lens may be constituted by a plurality of semi-cylindrical lens.
In the above-described laser irradiation apparatus, it is more preferable that the laser beam is a pulse laser having a frequency of 100 Hz or more.
Further, according to another structure of the present invention disclosed in the present specification, there is provided a laser irradiation method of carrying out irradiation while scanning a linear laser beam in a beam width direction, characterized in that the linear laser beam on an irradiation surface has a first energy density in a first beam width and a second energy density in a second beam width, and the second energy density is higher than the first energy density.
Further, according to another aspect of the present invention disclosed in the present application, there is provided a semiconductor device comprising a crystalline semiconductor film, characterized in that the crystalline semiconductor film has been irradiated with a linear laser beam having a first energy density in a first beam width and a second energy density higher than the first energy density in a second beam width on an irradiated surface, while having been scanned in a beam width direction.
Preferably, the semiconductor device is a thin film transistor including an active layer of the crystalline semiconductor film.
It is preferable that the thickness of the crystalline semiconductor film is 25 nm to 75 nm. If the film thickness is in this range, when the film is irradiated with the linear laser beam having the first energy density in the first beam width and the second energy density higher than the first energy density in the second beam width on the irradiation surface while being scanned in the beam width direction, in-plane uniformity of film quality is improved.
According to another aspect of the present invention disclosed in the present application, there is provided a method of manufacturing the semiconductor device, comprising the steps of: obtaining a crystalline semiconductor film by irradiating a non-single crystal semiconductor film on a substrate with a linear laser beam while the beam is scanned in a beam width direction, and manufacturing the semiconductor device by using the crystalline semiconductor film, characterized in that the linear laser beam on an irradiation surface has a first energy density in a first beam width and a second energy density in a second beam width, and the second energy density is higher than the first energy density.
In the above-described manufacturing method, it is preferable that the first beam width is equal to the second beam width.
In the above-described manufacturing method, scanning may be carried out from a side of the laser beam having the first energy density.
In the above-described manufacturing method, scanning may be carried out from a side of the laser beam having the second energy density.
In the above-described manufacturing method, a first scanning is carried out in a direction of the first energy density side of the beam width to said non-single crystal semiconductor film to anneal the film into a crystalline semiconductor film, and then, a second scanning is carried out in a direction of the second energy density side of the beam width after the first scanning.
In the above-described manufacturing method, scanning may be carried out from a side of the laser beam having the second energy density relative to the non-single crystal semiconductor film to anneal the film into a crystalline semiconductor film, and then, scanning is carried out from a side of the laser beam having the first energy density relative to the crystalline semiconductor film to carry out an annealing.
In the above-described manufacturing method, it is preferable that a difference between the first energy density and the second energy density is 4% to 30% of the first energy density.
In the above-described manufacturing method, it is preferable that irradiation is carried out in an atmosphere of one selected from the group consisting of He, Ar, N2 and a mixed gas of those.
In the above manufacturing method, it is preferable that the irradiation is carried out in the He atmosphere. By this, in the case where a cap layer etc. of a silicon oxide film etc. is not provided on the non-single crystal semiconductor film but the film is directly subjected to the laser irradiation, the in-plane uniformity of the film quality of the crystalline silicon film after the annealing is raised, and the phenomenon where the vicinity of a grain boundary of a crystal rises after the laser irradiation, called a ridge, is extremely lessened.
In the above manufacturing method, it is more preferable that the laser beam is a pulse laser with a frequency of 100 Hz or more. When the laser beam is made to have such a frequency, the scanning speed can be increased, so that the processing speed can be raised and the productivity of the semiconductor device is improved.